It's not slang for anything except someone far too proud of himself (yes, 'him'). And it's really not pronounced like 'coke'. It's cock but without the stress on both 'k' sounds, I don't know how to explain any other way than 'soft k'. It's also the name for eggshell, and if you've never had 'oeuf à la coque' with bread you're missing something (mostly an half-cooked egg eaten with a spoon, but what's the point of fully cooking an egg anyway...). Don't know if it factored in.

I'm not sure the pun is incidental, people like cute names with multiple interpretations. One other thing is that the rooster is kind of the national mascot. Here you'll hear someone shout 'cocorico' (rather mockingly) when some frenchman of woman realizes a feat. And there's a lot of self deprecation 'round here about the French being like roosters: feet in shit, and still blaring pride and moral lessons and thinking we're still a world power, to all who'll hear it. Our footballers have a rooster on their jersey I think, and you'll find many people here named Lecoq.

I think the 90s were a different, less PC time and the recent wave of indignation about schoolboys' jokes hadn't reached our shores, and most people here didn't speak that much English, even less the 'bad' words.

Not sure they expected the thing to become so popular then...